\ 548 Compressed Air. stood that the employment of fuel in a re-heater is attended with some six times the economy of the same fuel used in a steam boiler furnace. Figs. 563-4 shew at B an electrical re-heater formed of resistance coils in the circuit of the dynamo A, and at G a stove re-heater through which the air pipe passes. Still another saving is obtained by air injection. As heat is nothing but a form of work, it may be made to do work as soon as generated, instead of being allowed to dissipate. In Fig. 564 this is done by allowing the hot air to pass from the receiver D through the injector nozzle F, and thus an additional quantity of air is drawn into the cold receiver E to fill up the loss caused by shrinkage during cooling. The air being compressed to 100 Ibs. at a temperature of 484°, is reduced to 50 Ibs. in E, with a tem- perature of 201° y but the gain is certain, for the heat has been made to do work. Much mechanical improvement has been introduced in the compressors, such as the use of lever-lifted valves instead of air- moved flaps, avoiding wire-drawing. Clearance spaces have been much reduced, and the mains increased so as to bring the air velocity below 30 ft. per sec. Referring now to Fig. 561, the cylinders A A are compound high and low pressure, and the air enters first the suction valves F F of the cylinder B. Leaving by the valves E E, it passes by b to the surface condenser D, and then to the second cylinder c, which it enters by H H and leaves by G G. Finally it passes by pipe M to the storage receiver. The valves are lifted by levers P, moved by cams N on shafts d d. The Paris Compressed Air Company delivers about 8000 H.P. from two central stations, through thirty-five miles of piping, the further motor being 4! miles away. Prof. Kennedy measured the efficiencies in 1889, and found that for one I.H.P. in central engine, the customer received -39 I.H.P. with cold air and '47 with air re-heated just before entering his motor. With two- stage compression and other improvements, Frangois shewed in 1891 that a total efficiency of 46 could be reached with cold air, '65 with hot air, and *8 if the hot air was sprayed with water; which results have since been approached. This assumes efficiencies of compressor, main, and motor at "9, '96, and •93 respectively.